Peripheral arterial disease (PAD) is a potentially debilitating manifestation of systemic atherosclerosis affecting more than 200 million people worldwide. Accurate diagnosis is important for managing the disease and confers useful prognostic information. CT angiography (CTA) has become the preferred modality of vascular surgeons for the imaging evaluation of PAD, because of its simplicity and widespread availability. However, in addition to unwanted x-ray exposure from CTA, there is a nearly 40% prevalence of impaired renal function in patients with PAD. In such patients, iodine-based CT contrast agents pose a risk for contrast-induced nephropathy. As a result of efforts previously funded under NIH 1R01HL096916, the QISS nonenhanced MRA (NEMRA) technique was developed as a safer, simple push button nonenhanced alternative to CTA and contrast- enhanced MRA (CEMRA). However, no NEMRA technique has yet proven effective at 3 Tesla, which is widely considered the optimal field strength for CEMRA. In order to take advantage of the large signal-to-noise boost at 3 Tesla, severe image quality limitations relating to high specific absorption rate (SAR) and worsened B1 field homogeneity need to be overcome. This project aims to solve these challenges. It will improve patient outcomes by providing optimal selection of interventional treatment strategies irrespective of patient age, presence of diabetes, arrhythmia, metallic implants, or renal functional impairment. Moreover, it will greatly improve the safety profile of the imaging examination and avoid the need for renal function testing. This five-year project consists of an initial two-year technical development and optimization phase, followed by a two-site research study that will validate and ascertain the relative utility of 3T QISS MRA and peripheral CTA for the diagnosis and interventional management of PAD. Our specific aims are as follows: 1. To develop QISS techniques that are insensitive to magnetic susceptibility artifacts and B1 field inhomogeneity, and are optimized for the higher specific absorption rates encountered at 3 Tesla. These techniques will be evaluated in realistic flow phantoms, healthy subjects and patients with PAD. 2. To reduce the scan time for a complete QISS peripheral arterial study to less than five minutes so as to optimize procedural efficiency and patient comfort, while maintaining image quality and accuracy. 3. To develop ungated QISS techniques which match the image quality of ECG-gated QISS MRA. 4. To compare the diagnostic accuracy of QISS MRA at 3 Tesla (using techniques developed in Aims 1 and 2) for PAD compared with CTA, using digital subtraction angiography as the reference standard. A subsidiary aim is to compare UnQISS (developed in Aim 3) with CTA in patients with atrial fibrillation.